The prior art is replete with a variety of IMDs that provide diagnostic and/or therapeutic capabilities. Such IMDs include, without limitation: cardiac pacemakers; implantable cardioverters/defibrillators (“ICDs”); and various tissue, organ, and nerve stimulators or sensors. IMDs typically include functional components contained within a hermetically sealed enclosure or housing, which is sometimes referred to as a “can.” In some IMDs, a connector header or connector block is attached to the housing, and the connector block facilitates interconnection with one or more elongated electrical medical leads. The header block is typically molded from a relatively hard, dielectric, non-conductive polymer having a thickness approximating the thickness of the housing. The header block includes a mounting surface that conforms to, and is mechanically affixed against, a mating sidewall surface of the housing.
It has become common to provide a communication link between the hermetically sealed electronic circuitry of the IMD and an external programmer, monitor, or other external medical device (“EMD”) in order to provide for downlink telemetry transmission of commands from the EMD to the IMD and to allow for uplink telemetry transmission of stored information and/or sensed physiological parameters from the IMD to the EMD. As the technology has advanced, IMDs have become more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring, which in turn increase the variety of possible physiologic conditions and electrical signals handled by the IMD. Consequently, such increasing complexity places increasing demands on the programming system.
Conventionally, the communication link between the IMD and the EMD is realized by encoded radio frequency (“RF”) transmissions between an IMD telemetry antenna and transceiver and an EMD telemetry antenna and transceiver. The telemetry transmission system that evolved into current common use relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced by a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In some products, the RF carrier frequency is set at 175 kHz, and the prior art contains various RF telemetry antenna designs suitable for use in such applications. To support such products, the EMD is typically a programmer having a manually positioned programming head having an external RF telemetry antenna. Generally, the IMD antenna is disposed within the hermetically sealed housing; however, the typically conductive housing adversely attenuates the radiated RF field and limits the data transfer distance between the programmer head and the IMD RF telemetry antennas to a few inches. This type of system may be referred to as a “near field” telemetry system.
It has been recognized that “far field” telemetry, or telemetry over distances of a few to many meters from an IMD, would be desirable. Various attempts have been made to provide antennas with an IMD to facilitate far field telemetry. Many proposals have been advanced for eliminating conventional RF telemetry antenna designs and substituting alternative telemetry transmission systems and schemes employing far higher carrier frequencies and more complex signal coding to enhance the reliability and safety of the telemetry transmissions while increasing the data rate and allowing telemetry transmission to take place over a matter of meters rather than inches.
Telemetry antennas, whether designed for near field or far field operation, are susceptible to variations in the implanted environment (the IMD and antenna are surrounded by varying amounts of conductive body tissue when deployed). For example, a practical telemetry antenna will be designed to provide adequate gain, gain pattern, and bandwidth for the intended application. In this regard, a given antenna designed and tuned for operation with a subcutaneously implanted IMD may not perform effectively with a sub-muscularly implanted IMD (due to the increased gain requirements for a sub-muscle deployment). Furthermore, a given antenna designed and tuned for operation with a near field telemetry system may not perform effectively in a far field telemetry system. Consequently, it may be necessary for an IMD manufacturer to provide multiple versions of an IMD product, where each version has a different antenna architecture that is specifically designed to accommodate a particular implant location and/or telemetry system.
It remains desirable to provide an IMD telemetry antenna system that eliminates drawbacks associated with the IMD telemetry antennas of the prior art. In particular, it is desirable to have an interchangeable or optional telemetry antenna system for an IMD. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.